1. Field of the Invention
The present invention relates to a multi-resonant broadband antenna for use in a multiple frequency band, and more particularly, to a multi-resonant broadband antenna having a fractal structure.
2. Description of the Related Art
Examples of a currently used multi-band antenna include a planar inverted F antenna (PIFA) which has an antenna with a meander line structure or a stack type patch antenna employed as the radiation element. The conventional PIFA could be constituted in an inverted F shape on a antenna ground plane and divided into a feed line and a short-circuit which serves to short-circuit the radiation element of the PIFA from the antenna's ground plane, and shows a resonance characteristic which has a dependence on a distance between the feed line and the short-circuit, and shapes of the feed line and the short-circuit. In order to realize a multi-resonant characteristic in the PIFA, the radiation element above the PIFA is divided into portions having different sizes and then combined. In other words, several antennas having single band characteristics are combined and used. The PIFA may be realized as a small antenna having a multi-band characteristic. If several antennas are combined using a PIFA structure, radiation efficiency and gains of the antennas may be abruptly reduced. Accordingly, the PIFA may not be suitable to be used as a multi-band antenna producing three or more resonances.
When a meander line structure is employed as the antenna's radiating element, the resulting antenna shows similar characteristics to the PIFA. In other words, conventionally, a PIFA, meander line antenna, and several antennas having single band characteristics are often combined in order to realize a small multi-band antenna. If such a small multi-band antenna is realized to show multi-resonant characteristics however, radiation efficiency of the small multi-band antenna abruptly deteriorates. Thus, if the small multi-band antenna is used as a multi-resonant antenna producing three or more resonances, a problem may occur in the performance of the small multi-band antenna.
Also, a multi-resonant antenna using a stack type patch antenna structure includes radiation elements which have different sizes and are arrayed up and down. Accordingly, the size of the multi-resonant antenna is increased. Consequently, the multi-resonant antenna is not suitable to be used as a multi-band antenna producing three or more resonances due to a structural limit, like a PIFA and a meander line antenna as described above.
Contemporary designs for a monopole antenna employing a Hilbert grid structure, can have multi-band frequency characteristics; however, resonance frequency bands will be narrowly formed. Also, if the monopole antenna is made small, the resonance frequency bands become even narrower, and the efficiency of the monopole antenna is lowered. In addition, there are technical limitations impeding the design of monopole antenna, which restrict their suitably for use in a specific frequency band.